Climatic aridification and its driving mechanism in the Three-River Source Region over the past millennium

As hydroclimatic changes in the Three-River Source Region affect the streamflow of Asia's major rivers, i.e., the Yangtze, Yellow, and Mekong Rivers, downstream regions and nations face escalating threats to agriculture, biodiversity, and water security. Analyzing hydroclimatic changes during the late Holocene, as a modern warming analog, could provide key insights for forecasting water system responses to rising global temperatures. A general drying trend spanning the Medieval Warm Period to the Little Ice Age has been observed in both Yellow River and Mekong River Sourced Regions. However, the Yangtze River Source Region, which nearly covers half the Three-River Source Region, shows no conclusive evidence of this pattern, highlighting the urgency of further studies to resolve this spatial inconsistency. Here, with the assistance of proxies from organic carbon and carbonate contents, we used stable carbon and oxygen isotopes, and grain size distribution from a sedimentary core in Saiyong Co, to reconstruct regional hydroclimatic changes of the Yangtze River Source Region over the past 1700 years. Our new records reveal a regional aridity trend over the past millennium in the Yangtze River Source Region, consistent with records from surrounding regions. Specifically, Saiyong Co's hydroclimatic history is characterized by three distinct phases: relatively wet conditions from AD 200 to AD 1100, relatively dry conditions from AD 1100 to AD 1850, and a shift towards significantly wet conditions since AD 1850. By comparing these records with well-established climatic and tropical symbolic records, we probe the mechanism driving regional hydroclimatic changes across millennial trend and centennial timescales. The millennial drying trend can be largely attributed to the gradual weakening of the Indian Summer Monsoon, while centennial-scale hydroclimatic variations over the past millennium aligns with ENSO events. Since AD 1850, intensified monsoon precipitation and rising temperatures suggest a transition towards wetter conditions in the Yangtze River Source Region, a shift that demands further monitoring and investigation. These findings establish a vital baseline for evaluating climate change impacts on the Yangtze River Source Region's long-term ecological sustainability.